Process for in situ retorting of oil shale



Dec. 20, 1966 M. SLUSSER ETAL 3,292,699

PROCESS FOR IN SITU RETOHTING OF OIL SHALE Filed Aug. 10, 1964 ZOTZmOQ ZOTZMOQ zOlLmom HWELL 'dWELL HWELL v QI N Em

ATTORNEY United States Patent 3,292,699 PROCESS FOR IN SITU RETORTING OF OIL SHALE Marion L. Slusser, Arlington, and Dean P. Nichols,

Dallas, Tex., assignors t0 Mobil Oil Corporation, a

corporation of New York Filed Aug. 10, 1964, Ser. No. 388,384 Claims. (Cl. 166-11) This invention relates to recovering shale oil from oil shale deposits. More particularly, this invention relates to a process for recovering shale oil from such deposits by in situ retorting and combustion procedures.

Vast deposits of oil shales containing large quantities of a hydrocarbon-yielding material known as kerogen are found in the world, particularly in the United States. The kerogen is a resinlike material which in its native state is a solid. By retorting, the kerogen in the oil shale is converted into shale oil, a fluid similar to crude oil. In place retorting of the kerogen into shale oil by the heat generated through in situ combustion of some of the carbonaceous materials in the oil shale superficially appears practical. However, the physical character of oil shale, the temperature required for conversion of kerogen to shale oil and the resulting fuel requirements severely encumber the practical applications of in situ combustion procedures for retorting oil shale. For example, the flow of fluids in unretorted oil shale is principally by fractures. However, after oil shale is retorted free of shale oil, its permeability to gases increases sufficiently to permit an in situ combustion front to pass therethrough without dependence upon fractures for fluid flows. Additionally, oil shale in the natural state has a thermal conductivity coefficient substantially the same as fire brick.

In another aspect, with either direct or reverse in situ combustion procedures of conventional character, major amounts of the shale oil are consumed in the combustion front immediately upon their release from the oil shale. It will be apparent that normally only a very thin zone of retorting temperature exists immediately before the combustion front. One reason for this result resides in the low thermal conductivity of the oil shale. The shale oil is released by the heat from the combustion fronts in sufficiently close proximity to such front that great portions of these hydrocarbons are consumed before they can escape into the nearest fracture. Additionally, with either of the mentioned combustion procedures, large portions of the pyrolysis products of kerogen are in the form of noncondensable but combustible gases. The composition of such gases is such that at the present time they can be used commercially only after expensive treatments. These nonoondensable gases now are usually considered waste products.

In accordance with the process of this invention, successive combustion fronts moving in the same direction from a central location, first by inverse drive and then by one or more direct drives, are used to heat a portion of a deposit of shale oil to temperatures suitable for retorting oil shale. The last of the directly driven combustion fronts is continued outwardly from such heated portion into the next adjacent unburned portion of the deposit. The front-produced gases, preheated in the burned portion of the deposit, retort shale oil from the unburned portion of the deposit in advance of this front. The retor-ted shale oil escapes consumption in such front and is recovered in great quantities while the advancing front is sustained principally upon the residue remaining after retorting of the oil shale.

One object of this invention is to provide for retorting oil shale before an advancing direct-movement combustion front in a manner such that as kerogen is converted into shale oil, the shale oil is moved away from the combustion front and recovered. Another object is to provide a re-v torting process using in situ combustion from which substantial portions of the released shale oil can be recovered. Another object is to employ in such process a directmovement combustion front consuming only the residual coke and carbonaceous residue resulting from the retorting which produces shale oil. Another object is to utilize the noncondensable gases generated by combustion procedures to economic advantage in the in situ retorting of oil shale.

These and further objects will become apparent when read in conjunction with the following detailed description of the present invention, the appended claims, and the attached drawings, wherein:

FIGURE 1 is a plan view of a portion of an oil shale deposit penetrated by a plurality of boreholes through which the embodiments of the present invention may be practiced; and

FIGURES 2, 3, 4, and 5 are graphs illustrating the expected temperature profile taken along a vertical plane in the deposit shown in FIGURE 1 as a function of position during various steps of the present process.

In the present description the term shale oil is used to denote both liquid and gaseous hydrocarbons produced by retorting oil shale. The term noncondensable gases is used to denote the gas phase recovered among the fluids from the oil shale and separated in the field using conventional gas-oil separators.

Referring now to FIGURE 1, there is shown a portion of a deposit of an oil shale 11 to be re-torted, disposed within the earth, and assumed to be horizontally bounded by impervious strata. The oil shale 11, exclusive of any natural fractures, may be considered to have a permeability of air less than 1 millidarcy. After a substantial portion of the kerogen is removed by retorting from the oil shale 11, its permeability to air exclusive of any natural fractures, is expected to increase to between about 5 to 15 millidarcies. Such increased permeability to air will permit an in situ combustion front to traverse unfractured oil shale.

The oil shale 11 is penetrated, before or during the steps of this invention, to a desired depth by several boreholes, including a central borehole 12. Also provided is a first ring A of boreholes in surrounding relationship to the borehole 12. The ring A comprises boresholes 13, 14, 16, and 17. An annulus 18 residing between the borehole 12 and the ring A of boreholes is used as a heat transfer zone. For this purpose the annulus 18 has such dimensions, both as to thickness and horizontal extent, that when it is heated to temperatures suitable for retorting oil shale it has a sufficient quantity of sensible heat to retort the oil shale between the ring A and a surrounding ring B of boreholes by a gas passed through the annulus 18 in a direction radially outwardly from borehole 12. Usually, the first ring A of boreholes may be placed at a radius from about 5 to 50 feet about the central borehole 12. The second ring B of boreholes is disposed about the central borehole 12 outwardly from ring A at a similar spacing. Ring B includes the wells 19, 20, 21, 22, 23, 24, 25, and 26. The ring B of boreholes provides an annulus 27 circumferen tially surrounding the annulus 18. Additional surrounding rings of boreholes (not shown) may also be used. The boreholes may be completed in the usual manner for con veying fluids between the oil shale 11 and the earths surface. Although rings of boreholes are shown, it is to be understood that this process may be practiced between aligned boreholes, as, for example, boreholes 12, 14, and 24.

The oil shale 11 between the central borehole 12 and the rings of boreholes may in nature have in existence a highly developed interconnecting fracture or crevice system to provide a suitable permeability to air to permit in situ combustion. If not, the oil shale 11' can be exhaustively fractured to provide such permeability. Preferably, the oil shale 11 has particles ranging in size from about 1 inch to about 3 feet in diameter however they are created. Any means for fracturing the oil shale 11 may be used. For example, the oil shale 11 may be fractured hydraulically or by explosives. When the desired permeability exists for in situ combustion, the steps of the present process are practiced.

A reverse combustion front is passed from the borehole 12 to the ring A of boreholes as the first step in this invention. The following mechanisms may be used to practice this step. Combustion is initiated in the oil shale 11 adjacent to the central borehole 12. Any suitable means may be used for initiating combustion. For example, an electric heater disposed within the borehole 12 is energized to heat the adjacent surrounding oil shale 11 to ignition temperatures. A combustion-supporting gas is passed, via some or all of boreholes 13, 14, 16, and 17 in ring A, through the annulus 18 of the oil shale 11 and is vented from the central borehole 12. The combustionsupporting gas by inverse injection moves the reverse combustion front through the oil shale 11 from the central borehole 12 to the ring A of boreholes. The injection of the combustion-supporting gas through the boreholes of ring A is regulated so that the reverse combustion front moves outwardly at a uniform rate. As one result, this front traverses a substantial portion of the annulus 18 with a high vertical and areal sweep efliciency. Preferably, the combustion-supporting gas is held rich in oxygen so that the reverse combustion front consumes as much as possible of the combustibles in the annulus 18. This greatly increases the permeability to air of the annulus 18. When the reverse combustion front reaches the boreholes of ring A, the'injecting of the combustion-supporting gas is terminated. Closing-in of these boreholes in ring A at this time is preferred.

The reverse combustion front readily consumes a large portion of the combustibles in the annulus 18 and thereby greatly heats such annulus to high temperatures. However, it has been found in oil shales that in most circumstances approximately 23 to 27 percent by weight of a carbonaceous residue, as coke, is expected to remain in the annulus 18 even after such severe inverse burning. This residue will thereafter support the subsequent passage of approximately three separate combustion fronts.

As another step, a direct combustion front is passed through the annulus 18 in the oil shale 11 from the central borehole 12 to the ring A of boreholes. This step may be practiced by any desired technique. For example, the oil shale 11 adjacent the central borehole 12 is ignited by any suitable means such as those previously mentioned. A combustion-supporting gas is passed through the borehole 12 to flow radially outwardly into the oil shale 11. At this time, the boreholes in the ring A preferably are closed, except for monitoring purposes, and fluids are produced from the boreholes of the ring B. The resulting combustion front is moved from the central borehole 12 radially outwardly through the annulus 18. Preferably, the composition of the combustion-supporting gas, and its volume, are adjusted so that the effluent flue gases which move radially outwardly from the direct combustion front have an oxygen content insufliciently low to sustain combustion of combustible materials in the annulus 27 of the oil shale 11. For this purpose, noncondensable, but com- -bustible, gases which were vented from the central borehole 12 and stored in a suitable container or vented from the boreholes in ring B, may be intermixed with the combustion-supporting gas injected into the annulus 18. Samples of the efiluent flue gases may be taken from the boreholes of the ring A for monitoring purposes. Further, the conditions of injecting the combustion-supporting gas, particularly as to volume, may be adjusted for maintaining the efiiuent flue gases from the direct combustion front with the desired low oxygen contents. This result also can be promoted by moving the direct burning front at the minimum maintainable forward rate. Such combustion conditions can be determined mathematically from measurements of permeability, the amount of residual carbonaceous material therein, and the volume of the previously burned oil shale in the annulus 18. These combustion conditions can, of course, be determined experimentally;

from core samples or, on a larger scale, by a field trial. Usually, passage of one direct combustion front through the annulus 18 provides temperatures therein sufliciently high to retort the oil shale. Preferably, the direct combustion front passage through the annulus 18 is repeated for a suitable number of traverses until temperatures therein of over about 1200 F. are obtained. At such temperatures, mineral carbonates decompose and can result in greatly increasing the permeability to air of the annulus 18 in the oil shale 11. The resulting carbon dioxide reduces the combustion-supporting abilities of the eflluent flue gas from the direct combustion front. For purposes of monitoring the combustion front and its position, the procedures disclosed in U.S. Patents 2,800,183

and 3,044,543 may be utilized. Also, one or more of the boreholes in the ring A may be periodically vented for purposes of taking samples of the effluent flue gases from thedirect combustion front for inspection. It will be particularly noted that there is usually a direct visual notice of the arrival of the direct combustion front at any boreholes in the ring A open to the atmosphere. At such time, great volumes of smoke and tongues of fire may be emitted from such boreholes.

If desired, several direct combustion fronts may be passed in succession through the annulus 18 even though i the temperature therein is above the fusion temperature of the mineral matrix remaining of the oil shale 11. However, at such high temperatures, usually in excess of 2000 F., the continued injection of some gas through the annulus 18 is required to maintain permeability until the mineral matrix solidifies.

Thus, the gas injected from the borehole 12 into the oil shale 11 is heated to elevated temperatures before reaching the direct combustion front. As a result of such heating, the oxygen utilization in the combustion front is high with a corresponding oxygen reduction in the effluent flue gas. Upon passage through the front, the residue gas (effluent flue gas) is heated to even higher temperatures. Also, this gas has an oxygen content insufiicient to maintain combustion as a result of the foregoing steps. Thus, such heated gas will retort a portion of the oil shale 11 in the annulus 27 before this portion is reached by the.

direct combustion front. The volumetric capacity and the temperature of the annulus 18 are arranged such that the retorted portion, i.e., a retorting zone, in the annulus 27 is of suflicient dimension in front of the advancing direct combustion front that the released shale oil can escape from such combustion front and be driven by the efiluent flue gases to the ring B of boreholes for recovery without a substantial portion of such shale oil being consumed in the burning front.

The last of such direct combustion fronts moved through the annulus 18 is continued past the ring A of [boreholes radially outwardly through the annulus 27, to the boreholes of ring B at the same conditions as in the annulus 1-8. Thereafter, the front likewise may be continued to successive rings of boreholes which surround the ring B.

As the last step of this invention, fluids are produced from the boreholes in the ring B and the shale oil is recovered from such fluids by suitable apparatus. The noncondensable gases may be recycled through the direct combustion front to assist in maintaining the desired,

conditions in the eflluent flue gases discharged from such front.

If desired, after the direct combustion front has passed a substantial distance beyond the ring A of boreholes,

or the annulus 18 has in part decreased substantially below retorting temperatures, the borehole 12 may be closed-in and the combustion-supporting gas injected through the boreholes of the ring A. If desired, this procedure may be similarly extended outwardly in succession through adjacent rings of boreholes.

Referring now to FIGURES 2, 3, 4, and 5, a description of the expected temperature profiles taken along a line extending between the boreholes 12, 14, and 24 will be given. In these figures, curves 31, 32, 33, and 34 represent the expected temperatures along such line during various steps of the present process. In FIGURE 2, the curve 31 illustrates the temperatures with the reverse combustion front moving from the borehole 12 toward the ring A of boreholes. Referring now to FIGURE 3, the curve 32 illustrates the concentration of heat by such front when it arrives at the ring A, which includes the borehole 14. It will be noted that the curve 32 is substantially flat through most of the annulus 18 which indicates an equilibrium condition; The temperature represented by the curve 32 through the annulus 18 may be significantly above 700 F. for the combustion conditions described. Obviously, reverse combustion concentrates the greatest heat adjacent the ring A where it is needed in the greatest quantity for retorting the annulus 27.

Referring now to FIGURE 4, the curve 33 represents the temperature profile of the oil shale 11 when a direct combustion front has moved to a position midway between the boreholes 12 and 14 in the annulus 18. The peak temperature now is located at the direct combustion front. Behind this front, the temperatures of the an nulus 18 decrease as the relatively cool combustionsupporting gas arriving from the borehole 12 is heated to retorting temperatures. The temperatures before this front are slightly less in the annulus 18 than in the front as can be expected. For example, the temperatures in this front may be approaching the fusion temperature of the oil shale matrix. Beyond this front in the annulus 18, the minimum temperature may be closer to 1200 F. At this time, the efiluent flue gases passing beyond the ring A of boreholes will have retorted at least the first portion of the annulus 27, leaving a carbonaceous residue before the advancing front. The peak temperatures generated by reverse burning shown in the curve 32 at the borehole 14 now will have passed a short distance into the annulus 27 and represent a retorting zone.

In FIGURE 5, the curve 34 illustrates the temperature profile with the direct combustion front at a position about midway between the boreholes 14 and 24 in the annulus 27. The peak temperature generated in the direct combustion front may be above the fusion temperature of the mineral matrix of the oil shale 11. Behind this combustion front is the heat transfer zone which now extends through a portion of the annuli 18 and 27. It will be noted that a portion of the annulus 18 adjacent the central borehole 12 now may be cooled below retorting temperatures. The retorting zone extends over a substantial portion of the annulus 27 in front of the direct combustion front. This front may be considered to exist in that area of the curve 34 representing the peak temperatures which extends about A: the distance between boreholes 14 and 24. The retorting zone now would extend to a point approximately midway between the direct combustion front and the ring B, which includes the borehole 24. Thus, the retorting zone is of substantial thickness in the annulus 27 in front of the combustion front. This permits heated eflluent flue gases from the direct combustion front to retort the oil shale and for the shale oil to be moved to the boreholes of ring B for recovery. Since the direct combustion front is of rather narrow thickness in the annulus 2-7, as can be seen from the curve 34, and is sustained principally upon a carbonaceous residue, the amount of shale oil consumed by such front is greatly reduced.

As mentioned, the retorting of oil shale by hot gases leaves in place a residual carbonaceous material generally comprising a coke. This residual material is adequate to sustain a direct combustion front. After the oil shale hasbeen retorted, its permeability is greatly increased. The conditions under which the direct combustion front traverses the annulus 27 are such that the front does not increase in thickness even though before its permeability has been increased in the oil shale. One reason for this result is that the amount of available oxygen supplied to such combustion front is limited to control effectively the rate of combustion of the residual carbonaceous materials. Thus, equilibriumbetween the heat transfer, combustion, and retorting zones is established adjacent the ring A in the annulus 27 and these zones will be retained as the direct combustion front moves toward the boreholes beyond the ring A.

It will be apparent from the foregoing that there has been provided a process well suited for satisfying all of the objects of this invention. Those skilled in the art will be able to make use of certain combinations of steps of this invention to obtain previously unobtainable results. Also, various changes may be made to these steps, and to their order, without departing from the scope or intent of this invention. It is intended that such changes, alterations, and modifications be encompassed within the scope of the appended claims, which claims contain the only limitations to be applied to this invention.

We claim:

1. A process for recovering hydrocarbons from a deposit of oil shale penetrated by boreholes, comprising the steps of:

(a) passing a combustion front through the deposit from a first borehole to a second borehole by the inverse injection of a combustion-supporting gas through the deposit between the boreholes,

(b) passing a combustion front for a suitable number of traverses through the deposit from the first to the second borehole by the direct injection of a combustion-supporting gas through the deposit residing between boreholes until there is produced a mass of burned deposit at temperatures suitable for retorting oil shale,

(c) passing the last of such combustion fronts by direct combustion on to a third borehole disposed in an unburned portion of the deposit beyond the second borehole by the direct injection of a combustion-supporting gas through the deposit residing between the boreholes, and

(d) producing fluids from the third borehole and recovering hydrocarbons from such fluids.

2. The process of claim 1 where the injection of the mentioned gas is adjusted to provide an efiluent flue gas discharged from the combustion fronts of steps (b) and (c) into the deposit between the second and third boreholes with an insufficient oxygen content to support further combustion.

3. The process of claim 1 where the mass of the deposit swept by the fronts of steps (a) and (b) has a suflicient heat content to retort the mass of the deposit to be swept by the front of step (c) substantially free of shale oil leaving only a carbonaceous residue remaining in such deposit.

4. A process for recovering hydrocarbons from a deposit of oil shale penetrated by boreholes, comprising the steps of:

(a) passing a combustion front through the deposit from a first borehole to a second borehole by the inverse injection of a combustion-supporting gas through the deposit residing between the wellbores,

(b) regulating the injection of such gas to burn a substantial portion of the combustibles in the deposit residing between the boreholes,

(c) passing a combustion front for a suitable number of traverses through the deposit from the first borehole to the second borehole by the direct injection of a combustion-supporting gas through the deposit residing between boreholes to produce a mass of burned deposit at temperatures suitable for retorting oil shale,

(d) passing the last of such combustion fronts by direct combustion on to a third borehole disposed in an unburned portion of the deposit beyond the second borehole by the direct injection of a combustionsupporting gas through the deposit residing between boreholes,

(e) adjusting the injection of such gas recited in steps (c) and (d) to provide an effluent flue gas discharged from the combustion fronts into the deposit between the second and third boreholes with an insufiicient oxygen content to support further combustion in the unburned portion of the deposit, and

(f) producing fluids from the third borehole and recovering hydrocarbons from such fluids.

5. A process for recovering hydrocarbons from a deposit of oil shale penetrated by boreholes, comprising.

the steps of:

(a) initiating combustion in said deposit adjacent a first borehole, I

(b) injecting a combustion-supporting gas through a second borehole into said deposit to move a combustion front from the first borehole to the second borehole,

(c) regulating the injecting of the mentioned gas to burn a substantial portion of the carbonaceous deposit,

(d) terminating the injecting of the mentioned gas through the second borehole when the combustion front arrives,

(e) initiating combustion in said deposit adjacent said first borehole,

(f) injecting a combustion-supporting gas through the first borehole into the deposit to move a combustion front from the first borehole to the second borehole for a suitable number of traverses to produce a mass of depleted oil shale at temperatures suitable for retorting oil shale,

(g) passing the last of such combustion fronts by direct combustion on to a third borehole disposed in an unburned portion of the deposit beyond the second borehole by the direct injection of a combustionsupporting gas through the deposit residing between boreholes,

(h) adjusting the injecting of the mentioned gas to produce an efiiuent flue gas passing from the combustion fronts in steps (f) and (g) intothe deposit residing between the second and third boreholes with an insufl'icient oxygen content to support further combustion of the combustibles in the unburned deposit between the second and third boreholes which resides in advance of the combustion front, and

(i) producing fluids from the third borehole and recovering hydrocarbons from such fluids.

6. A process for recovering hydrocarbons from a deposit of oil shale penetrated by boreholes, comprising the steps of:

(a) passing a combustion front through the deposit from a central borehole to a surrounding first ring of boreholes by the inverse injection of a combustion-supporting gas through the deposit residing between the central borehole and the first ring of boreholes,

(b) passing a second combustion front in a suitable num er of traverses through the deposit from the central boreholeto the first ring of boreholes until a mass of said deposit is depleted of combustibles and reaches temperatures suitable for retorting oil shale by the direct injection of a combustion-supporting gas through the deposit residing between boreholes,

(c) passing the last of such combustion fronts by direct combustion on to a surrounding second ring of boreholes disposed in an unburned portion of the deposit beyond the first ring of boreholes by the direct injection of a combustion-supporting gas,

through the deposit residing between the'boreholes, and

(d) producing fluids from the second ring of bore holes and recovering hydrocarbons from'such fluids.

7. The process of claim 6 wherein steps (b) and (c), the injection of the mentioned gas is adjusted to provide an efiluent flue gas discharged from the combustion fronts in the previously burned deposit into the unburned deposit between the second and third boreholes with .an insufficient oxygen content to support further combustion.

8. The process of claim 6 where the mass ofthe depleted deposit at a high temperature produced in the step (b) has a sufficient heat content to retort the mass of the unburned deposit residing between the first and second rings of boreholes substantially free of shale oil in advance of the front of step (c) 9. A process for recovering hydrocarbons from a deposit of oil shale penetrated by boreholes, comprising the steps of:

(a) passing a first combustion front through the deposit from a central borehole to a first ring of boreholes by the inverse injection of a combustion-supporting gas through the deposit residing between boreholes, (b) regulating the injection of such gas to burn a substantial portion of the combustibles in the deposit between the central borehole and the first ring of the direct injection through the deposit of a com? bustion-supporting gas,

(e) adjusting the injection of such last-mentioned gas to provide an efiluent flue gas discharged from the combustion fronts of steps (0) and ((1) into the deposit residing beyond the first ring of boreholes with an insufficient oxygen content to support furthedr combustion in advance of the combustion front, an

(f) producing fluids from the second ring of boreholes and recovering hydrocarbons from such fluids.

10. A process for recovering hydrocarbons from a deposit of oil shale penetrated by boreholes, comprising the steps of:

(a) initiating combustion in said deposit adjacent a central borehole, (b) injecting a combustion-supporting gas through a surrounding first ring of boreholes in said deposit to move a combustion front from the central bore-, t

hole to the first ring of boreholes,

(c) regulating the injecting of the mentioned gas to burn a substantial portion of the combustibles in the deposit,

(d) terminating the injecting of the mentioned gas and closing-in the first ring of boreholes when the combustion front arrives,

(e) initiating combustion in said deposit adjacent the central borehole and injecting a combustion-supporting gas through such borehole to move a combustion front from such central borehole to the first ring of boreholes in a suitable number of traverses until the burned portion of the deposit within the first ring of boreholes has a sufiicient heat content for retorting a portion of the deposit between the first ring of boreholes and a surrounding second ring of boreholes disposed in an unburned portion of the deposit and then moving the last of said combustion fronts by direct combustion outwardly toward the second ring of boreholes,

(f) adjusting the injecting of the last-mentioned gas to provide an efiiuent flue gas discharged from the combustion fronts of step (e) into the deposit between the first and second rings of boreholes Withan insufiicient oxygen content to support further combustion of the combustibles in the deposit in advance of the combustion front between these rings of boreholes, and (g) producing fluids from the second ring of boreholes and recovering hydrocarbons from such fluids.

References Cited by the Examiner UNITED STATES PATENTS CHARLES E. OCONNELL, Primary Examiner.

S. I. NOVOSAD, Assistant Examiner. 

1. A PROCESS FOR RECOVERING HYDROCARBONS FROM A DEPOSIT OF OIL SHALE PENETRATED BY BOREHOLES, COMPRISING THE STEPS OF: (A) PASSING A COMBUSTION FRONT THROUGH THE DEPOSIT FROM A FIRST BOREHOLE IN A SECOND BOREHOLE BY THE INVERSE INJECTION OF A COMBUSTION-SUPPORTING GAS THROUGH THE DEPOSIT BETWEEN THE BOREHOLES, (B) PASSING A COMBUSTION FRONT FOR A SUITABLE NUMBER OF TRAVERSE THROUGH THE DEPOSIT FROM THE FIRST TO THE SECOND BOREHOLE BY THE DIRECT INJECTION OF A COMBUSTION-SUPPORTING GAS THROUGH THE DEPOSIT RESIDING BETWEEN BOREHOLES UNTIL THERE IS PRODUCED A MASS OF BURNED DEPOSIT AT TEMPERATURES SUITABLE FOR RETORTING OIL SHALE, (C) PASSING THE LAST OF SUCH COMBUSTION FRONTS BY DIRECT COMBUSTION ON TO A THRID BOREHOLE DISPOSED IN AN UNBURNED PORTION OF THE DEPOSIT BEYOND THE SECOND BOREHOLE BY THE DIRECT INJECTION OF A COMBUSTION-SUPPORTING GAS THROUGH THE DEPOSIT RESIDING BETWEEN THE BOREHOLES, AND (D) PRODUCING FLUIDS FROM THE THIRD BOREHOLE AND RECOVERING HYDROCARBONS FROM SUCH FLUIDS. 